To get in line with the global trend of environmental protection and eco-friendly regulations, electronic product manufacturers nowadays are developing and manufacturing halogen-free electronic products. Advanced countries and electronic manufacturing giants set forth schedules of launching mass production of halogen-free electronic products. As a result of the promulgation of the Restriction of Hazardous Substances (RoHS) by the European Union, hazardous substances, such as lead, cadmium, mercury, hexavalent chromium, poly-brominated biphenyl (PBB), and poly-brominated diphenyl ether (PBDE), are strictly prohibited from being used in manufacturing electronic products or their parts and components. A printed circuit board (PCB) is an indispensable and fundamental basis of the semiconductor industry and electronic industry; hence, printed circuit boards bore the brunt of international halogen-free regulations when international organizations set forth strict requirements of the halogen content of printed circuit boards. For example, the International Electrotechnical Commission (IEC) 61249-2-21 requires that bromine content and chloride content shall be less than 900 ppm and the total halogen content shall be less than 1500 ppm. The Japan Electronics Packaging and Circuits Association (JPCA) requires that both bromide content and chloride content shall be less than 900 ppm. To enforce its green policies, Greenpeace calls on manufacturers worldwide to get rid of polyvinyl chloride (PVC) and brominated flame retardants (BFRs) from their electronic products in order to conform with the lead-free and halogen-free requirements of green electronics. Hence, the industrial sector nowadays is interested in rendering related materials halogen-free and sees this technique as one of its key research topics.
Electronic products nowadays have the trend toward compactness and high-frequency transmission; hence, circuit boards nowadays typically feature a high-density layout and increasingly strict material requirements. To mount high-frequency electronic components on a circuit board, it is necessary that the substrate of the circuit board is made of a material of a low dielectric constant (Dk) and dielectric dissipation factor (Df) in order to maintain the transmission speed and the integrity of a signal transmitted. To allow the electronic components to function well at a high temperature and a high-humidity environment, it is necessary for the circuit board to be heat resistant, flame resistant, and of low hygroscopicity. Epoxy resin is adhesive, heat resistant, and malleable and thus is widely applicable to encapsulants and copper clad laminates (CCL) of electronic components and machinery. From the perspective of fire prevention, epoxy resin is incapable of flame retardation, and thus epoxy resin has to acquire flame retardation capability by including a flame retardant therein. For example, a halogen, such as bromine, is included in epoxy resin to not only bring about flame retardation capability thereof but also enhance epoxy reactivity. However, electronic products developed in recent years have a trend toward being lightweight, compact, and circuit-miniaturized. Due to the aforesaid requirement, halides which have a large specific gravity are undesirable from the perspective of weight reduction. Furthermore, after long use, halides are likely to decompose at high temperature, which often results in corrosion of fine circuits. Also, upon their combustion, discarded electronic parts and components produce halides which are most hazardous and environmentally unfriendly. To find an alternative to the aforesaid halide-based flame retardant, researchers attempt to use a phosphorous compound as a flame retardant, for example, adding phosphate ester (U.S. Pat. No. 6,440,567) or red phosphorus (EP 0763566) to an epoxy resin composition. However, phosphate ester undergoes hydrolysis readily to produce an acid, thereby compromising its tolerance to migration. Although red phosphorus is good at flame retardation, it falls into the category of hazardous compounds under the firefighting law, because it produces a trace of a flammable, toxic gas known as phosphine in a humid environment at high temperature.
U.S. Pat. No. 7,255,925 discloses a thermosetting resin composition composed of cyanate ester resin, dicyclopentadiene (DCPD) epoxy resin, silica, and a thermoplastic resin. The thermosetting resin composition is characterized by a low dielectric constant (Dk), and a low dielectric dissipation factor. However, a method for manufacturing the thermosetting resin composition of U.S. Pat. No. 7,255,925 requires the use of a halogen-containing (such as bromine-containing) flame retardant, such as tetrabromocyclohexane, hexabromocyclodecane, or 2,4,6-tris(tribromophenoxy)-1,3,5-triazine. However, the bromine-containing flame retardant causes environmental pollution readily during the thermosetting resin composition manufacturing process, the using processing of thermosetting resin composition, and even after the thermosetting resin composition has been discarded or recycled. To ensure a low dielectric dissipation factor, low hygroscopicity, high cross-linking density, high glass transition temperature, high connectivity, appropriate thermal expansion, heat resistance, and flame retardation of metal clad laminates, an important factor lies in the selection of an epoxy resin, a curing agent, and a reinforcement material.
At present, to enable environment-friendly halogen-free resin compositions to attain UL94 V-0 flame retardation, it is usually necessary to add thereto a phosphorus-containing flame retardant. The phosphorus-containing flame retardant preferably contains a phosphazene. However, a conventional phosphazene (such as SPB-100 manufactured by Otsuka Chemical Co., Ltd. (hereinafter “Otsuka Chemical”)) lacks a reactive functional group, and, as a result, the conventional phosphazene contained in a halogen-free resin composition cannot react with any other resin. As a result, a laminate manufactured from the halogen-free resin composition has an overly large coefficient of thermal expansion, thereby causing a circuit board manufactured from the laminate to crack internally during a manufacturing process and reducing the yield. In view of this, phosphazene suppliers further developed a phosphazene having a hydroxyl group (such as SPH-100 manufactured from Otsuka Chemical). Due to its hydroxyl group, SPH-100 reacts with any other resin readily. However, SPH-100 has a disadvantage, that is, its hydroxyl group results in overly high dielectric constant (Dk) and overly high dielectric dissipation factor (Df).
The major considerations given to electrical properties include the dielectric constant and the dielectric dissipation factor. In general, the signal transmission speed of a laminate is inversely proportional to the square root of the dielectric constant of the material from which the laminate is made, and thus the minimization of the dielectric constant of the laminate material is usually advantageously important. The lower the dielectric dissipation factor is, the lesser the signal transmission attenuation is; hence, a material of a low dielectric dissipation factor provides satisfactory transmission quality.
Accordingly, it is important for printed circuit board material suppliers to develop materials of a low dielectric constant and a low dielectric dissipation factor, and apply the materials to high-frequency printed circuit board manufacturing.